Circumcised men are much less likely than uncircumcised men to become infected with the AIDS virus, according to a new study published in Britain. The finding provides support to those who say that biology rather than behavior is a major factor in the decreased risk of HIV infection in circumcised men. But opponents of male circumcision are not persuaded that the procedure has any health benefits.

Scientists have long noted that circumcised men appear less likely than uncircumcised males to become infected with HIV, the virus that causes AIDS. The explanation for this, some believed, was somehow linked to behavior: There are fewer AIDS cases in countries where circumcision is prevalent, because circumcised males must be doing a better job of protecting themselves against infection.

But a new study by U.S. and Indian researchers disputes this theory.

Investigators followed 2,300 men at three clinics in Pune, India. At the beginning of the study, all of the men were HIV-negative, according to Bob Bollinger, a professor of infectious diseases at Johns Hopkins University in Baltimore, Maryland.

At the end of seven years, circumcised men were 6.5 percent less likely than uncircumcised males to be infected with the AIDS virus. But they had the same rates of other sexually transmitted diseases, including syphilis and gonorrhea, as males who still had their foreskins.

Professor Bollinger says the results are not surprising based on other studies. But what's different about the Lancet study, says Mr. Bollinger, is that researchers are now offering a clue as to why circumcision is protective.

The Indian researchers took their findings to the lab and found that the foreskin contains cells that promote HIV infection. "These are cells that have a molecule called CD-4 on their surface which is essentially a magnet, if you will, for HIV infection," says Professor Bollinger.

But not everyone agrees.

"We are carrying out a non-medically necessary procedure on an individual who is unable to consent to it," says Arif Bhimiji, an emergency room doctor in Canada who opposes circumcision. He first became interested in the operation when he saw infected infants brought into a Toronto hospital.

"This procedure has known risks. It is irreversible," he says. "And we should not be using this medical evidence as a justification for carrying out a procedure that really is of no known significant benefit to the child at that particular point in their life." Dr. Bhimji says he knows of no infant who has ever become infected with the AIDS virus through sexual contact. He believes a decision about whether to have a circumcision should made by men when they become sexually active.

Both experts agree on one thing, though. Education about the use of condoms and the risk of unsafe sex are key to preventing the transmission of HIV.

"Circumcision by itself is not sufficient to reduce your risk of HIV infection," says Professor Bollinger.

The study on circumcision and HIV transmission is published in The Lancet, a British medical journal.